[dv] Make mem_model's compare_byte function less chatty

[rswarbrick]

This spits out quite an impressive ream of text, making --verbosity=medium less useful than it might be.

Looking at the history of the file, I think this message got promoted from UVM_HIGH to UVM_MEDIUM as part of commit 39b2692. I suspect this was a debug print that survived by accident.

Demote it again!